VELOCITY OF WHITE AND OF COLOURED LIUHT. 
237 
If E B =pE A this becomes 
a(l-K)+r-^= # .{a(l-(0-r+^} 
If p, k and k were known we could determine the values of N A and N B from 
this equation when the velocity n giving equality of brightness had been determined, 
and when the distances D A and D B were known (for But these quantities, 
V LJ B ±J A J 
p, k, and k, are not known. 
5. Let us now consider the T th equality, when r has the same value as before. 
Here B is decreasing, A is increasing. We have p A =r-\- 2, Ps=r+ 1 . 
Ia=Ib ; or 
E', 
j 2 (l- K )_( ) .+2)+Y}= E ^{2(l- K ') + (r+l)-l 
n‘ 
'N 
B. 
Since E' B still =pE / A , this becomes 
2(1 _ K )_(r+a)+A= / ,{a(l - K ')+r- A 
6. From these two measurements, viz., of n and nf, we can determine N A and N B in 
D 
special circumstances. Let —=p. Then we have two equations 
2 ( l-K)+r-|-=p{2(l- K Vr+-jy 
2(l- K )-(r+2)+^-=p{2(l- K ') + >’-^} 
Subtracting, we have 
2(r+1 +pr)=±-J^+ l^(n+n)=^(n+n') 
If now the distances D A and D B have been carefully chosen so as to make g: 
this equation becomes 
2 r(g+p)= g ~?(n+n') 
and 
n + n' 
r + 1 
r 
n b =- 
2 r 
From this we can deduce the. velocity of light. For 
